1. Field of the Invention
The present invention relates to a video image mixing apparatus for mixing a playback video signal obtained by playing a recording medium such as a video disc or the like and a graphics video signal produced on the basis of image data read-out from a memory device.
2. Description of Background Information
The color video signal of the NTSC system is subjected to various limitations because considerations are given to maintain the compatibility so that a video image and audio signals are reproduced with a monochrome (black and white) television receiver as well. For example, the frequency of a chroma signal is selected to a sufficiently high frequency of about 3.58 MHz, in order to minimize the interference with a luminance signal and to enable the reproduction of the original color information. Specifically, assuming that the frequency of a horizontal sync signal is labelled as f.sub.H, the frequency f.sub.SC of the chroma signal is given as follows. EQU f.sub.SC= f.sub.H .times.455/2
Namely, since the frequency f.sub.SC is odd number times of one half the frequency of the horizontal sync signal, the phase of the chroma signal is inverted every frame.
A color video signal recorded on the recording medium such as a video disc or the like also has a chroma signal whose phase is inverted every frame in accordance with the above-described standard. Consequently, when the recording medium is played, the reproduction in the ordinary reproducing mode is performed to produce a color video signal in which the continuity in phase alternation of the chroma signal is maintained. On the other hand, in a special playing mode e.g., still image reproducing mode, there is a case where the video signal of the same frame is repetitively reproduced or the video signal consisting only of the odd or even number frames is continuously reproduced in a double-speed reproducing mode. In such a special playing mode, the video signal having the chroma signal of the same phase is continuously reproduced and the chroma signal whose phase is inverted every frame cannot be provided, so that color synchronization of a resultant video image is distorted, to make the video image colorless.
In a conventional apparatus of a video disc player or the like, therefore, a delay line having a delay time of about 140 nsec (.apprxeq.1/(2.times.3.58 MHz)) corresponding to one-half the period of the chroma signal is arranged on a video signal line. Therefore if the discontinuity of the phase alternation of a color burst signal occurs in the special playing mode, the video signal is delayed by about 140 nsec (nanoseconds) for every other frame, so that the phase shift is executed.
In cases where a video signal of a still video image (hereinafter, referred to as a still image video signal) obtained by playing the recording medium by such a conventional apparatus in the special playing mode is mixed (combined) with a video signal of a graphics video image (hereinafter, referred to as a graphics video signal), since the graphics video signal is produced from digital data, the latter signal is obtained as a stable video signal including no time fluctuation component. However, since a frame that is delayed by about 140 nsec exists in the still image video signal mentioned above, there exists a problem that a CRT display apparatus which receives the mixed video signal is subjected to the influence by the time fluctuation of the input signal. Thus, a fluctuation of video image on a display screen is produced whichever video signal the display apparatus traces.